Democratic U.S. Sen. Mark Pryor and Republican U.S. Rep. Tom Cotton have advanced without opposition to November's general election for a U.S. Senate seat.

Neither Pryor nor Cotton faced opposition within their own parties in Tuesday's primary election. The race between Pryor and Cotton is one of the most expensive and closely watched Senate contests in the country.

Outside groups already have spent millions of dollars in advertising in the state. Pryor, the only Democrat in the state's congressional delegation, was first elected in 2002 and is seeking a third term. Cotton is a freshman congressman representing south Arkansas' 4th District and launched his bid to unseat Pryor in August.

The two face Green Party nominee Mark Swaney and Libertarian Nathan LaFrance in the general election.